1. Field of the Invention
The present invention relates to non-oriented electrical steel sheets and a method for producing non-oriented steel sheets, and more particularly to compositions of non-oriented electrical steel sheets and the conditions for hot-rolling thereof.
2. Description of the Prior Art
Non-oriented electrical steel sheets are widely used for core materials of electrical apparatus for example, a rotating machine. Recently, for increasing the efficiency of, reducing the weight of and compacting these electrical apparatuses, materials having low core loss and high magnetic flux density have been in demand.
Steel sheets to which silicon is added, so-called "silicon steel sheets," have been customarily used as non-oriented electrical steel sheets. The addition of Si to steel increases specific resistance and reduces core loss value. However, because Si is an element having a characteristic of allowing the .alpha.-phase to be stabilized as shown in FIG. 1, the Ar.sub.3 transformation point temperature of silicon steel is raised in compliance with addition of Si, and the .gamma.-phase of the silicon steel closes its loop when the addition of Si reaches a certain amount. The .gamma.-phase of extra low carbon steel which contains no Al closes its loop at approximately 1.7 wt % Si, while the critical Si-amount is decreased when Al is added to the extra-low carbon steel. Changes of Ar.sub.3 transformation point temperatures in a range of 800.degree. to 1,000.degree. C. meet finishing temperatures at hot rolling. Therefore, hot rolling in the whole length at the Ar.sub.3 transformation temperature range becomes more difficult as the Si addition amount is increased. That is to say, in the case of a steel containing 1.7 wt % Si as shown in FIG. 1, the Ar.sub.3 transformation point temperature reaches 900.degree. C. and more. For this reason, conventional methods do not permit finishing hot-rolling temperatures above their Ar.sub.3 transformation points.
To overcome the difficulty the art has been forced to adopt high temperature heating. However, the means for heating Si contained steel sheets at high temperatures of 1,200.degree. C. and more has a disadvantage in that the surface smoothness property of the Si contained steel sheets is deteriorated. This is because, when the silicon contained steel sheets are heated at high temperatures of 1,200.degree. C. and more, slab surface scales are melted, exfoliative features of the slab surface scales before hot rolling are lowered, and scales rolled-in during the process of hot rolling.
Moreover, even if the finishing temperature is maintained at the Ar.sub.3 transformation point or more, by lower temperature heating, the means still has a drawback that the magnetic property of the final products deteriorates, because, in this case, owing to edge portions of steel slabs being hot-rolled in the state of having ferrite and austenite dual phases, the thickness and structure of the edge portions of hot-rolled steel sheets become non-uniform, due to difference of deformation resistance of the two phases.